Rubber compositions including those containing a coupler, or carbon black treated with a coupler, are described. The use of carbon black couplers has been hindered by the low number of functional groups on carbon black fillers that can potentially react with or otherwise bind to a coupler. A coupler would generally be expected to increase the interaction between the filler and an unsaturated rubber.
Carbon black has been the reinforcing filler of choice for many years for tires and other rubber applications, which need toughness (high tensile and tear strength), wear resistance, long fatigue life, and variable hysteresis rubber compositions. Recently silica, when used with a silane coupler, has been used to replace all or part of the carbon black in some tire formulations. The entry of silane coupler/silica systems in tires has been motivated by the potential for reduced rolling resistance in the tires, which can increase fuel economy, along with retaining good wet skid resistance. To date, similar performance enhancements have not been economically possible with all-carbon black filled compositions.
A group of couplers (carbon black coupling agents) is described which can either improve the dispersibility of the carbon black or improve the bonding of the rubber to the carbon black or combinations thereof. The compounds are referred to as couplers because they have at least one amine group that can react with an acid group on the surface of the carbon black and either 1) a polysulfidic linkage, which can during vulcanization fracture and form a chemical bond to an unsaturated rubber, or 2) a thiol group, which can eventually form a chemical bond to an unsaturated rubber. Amine-bearing compounds have been shown to bind to the carbon surface, by reacting with strong acid groups. (S. D. Gardner, Applied Spectroscopy, 51 (5), 636, 1997). In some embodiments, the carbon black used has been chemically oxidized to increase the number of acidic groups. The increase in acidic groups tends to enhance the performance of the coupler but is not required in some applications. The couplers generally have one or more amine terminal groups and one or more ester or amide linkages in them. The coupler tends to decrease the tan delta of the final rubber compound at 60xc2x0 C., which is associated with high fuel economy/lower rolling resistance, with less reduction in the tan delta at 0xc2x0 C., which is associated with wet traction. A large inversion ratio (tan delta 0xc2x0 C./tan delta 60xc2x0 C.) is usually indicative of an improvement in tire performance when judged on the basis of hysteresis properties and wet traction. When compared with silica-based compositions, carbon black compounds are easier to mix and have lower costs due to the filler cost differences. Hence, the industry has been seeking carbon black formulations that approach silica in hysteresis performance (e.g. large inversion ratio). Rubber compositions including carbon black, the coupler, and an unsaturated rubber are useful in tires, e.g. tread compounds, and other rubber articles where it would be desirable to control the interaction between the carbon black surface and the rubber. These would include hysteretic properties but could also include other physical properties of the rubber such as wear resistance, tear strength etc. Other rubber articles where hysteretic properties are important include vibration damping devices and other devices where heat buildup from deformation or stress may cause reversion and consequently failure of the part.